1. Field of the Invention
The present invention relates generally to methods for stabilizing plasma processing apparatuses and systems. In particular, the present invention relates to a technology suitable for reducing the feeding loss of the RF power.
2. Description of the Related Art
FIG. 42 illustrates an example of a conventional dual-frequency excitation plasma processing apparatus which performs a plasma process such as chemical vapor deposition (CVD), sputtering, dry etching, ashing, or the like.
The plasma processing apparatus shown in FIG. 42 comprises an RF generator 1, a plasma excitation electrode 4, and a matching circuit 2A disposed between the RF generator 1 and the plasma excitation electrode 4. The matching circuit 2A performs impedance matching between the RF generator 1 and the plasma excitation electrode 4.
RF power supplied from the RF generator 1 is fed to the plasma excitation electrode 4 through a supply line 1A and the matching circuit 2A. The matching circuit 2A is connected to the plasma excitation electrode 4 via a feed plate 3. Alternatively, the matching circuit 2A may be connected to the plasma excitation electrode 4 via the feed plate 3 and a feed line 3A. The supply line 1A and the feed line 3A are coaxial cables. The matching circuit 2A is housed in a matching box 2 made of a conductor. The feed plate 3 and the plasma excitation electrode 4 are covered with a chassis 21 made of a conductor.
The plasma excitation electrode 4 has an annular projection 4a on the bottom face. A shower plate 5 having a number of holes 7 is provided under the plasma excitation electrode 4 and is in contact with the projection 4a. The plasma excitation electrode 4 and the shower plate 5 define a space 6. A gas feeding tube 17 made of a conductor is connected to the space 6. The gas feeding tube 17 is provided with an insulator 17a which insulates the plasma excitation electrode 4 from the gas source.
In this power supply section, as shown in FIG. 38, the supply line 1A, which is a coaxial cable, generally has an extra length so as to increase the flexibility of installing the matching box 2 and a plasma processing chamber 60 described below. The supply line 1A is either wound and placed on a floor GF or routed along a ceiling GC, as shown in FIG. 38.
Referring to FIG. 42, gas is fed to the interior of the plasma processing chamber 60, surrounded by a chamber wall 10, through the holes 7 of the shower plate 5. An insulator 9 is disposed between the chamber wall 10 and the plasma excitation electrode 4 (cathode) to provide insulation therebetween. The exhaust system is omitted from the drawing.
A wafer susceptor (susceptor electrode) 8, which holds a substrate 16 and also serves as another plasma excitation electrode, is installed in the plasma processing chamber 60. A susceptor shield 12 is disposed under the wafer susceptor 8.
The susceptor shield 12 comprises a shield-supporting plate 12A for supporting the susceptor electrode 8 and a cylindrical supporting tube 12B extending downward from the center of the shield-supporting plate 12A. The supporting tube 12B penetrates a chamber bottom 10A, and the lower portion of the supporting tube 12B and the chamber bottom 10A are hermetically sealed with bellows 11.
The gap between the susceptor electrode 8 and the susceptor shield 12 is vacuum-sealed and electrically isolated by insulation means 12C composed of an insulating material provided at the periphery of a shaft 13. The susceptor electrode 8 and the susceptor shield 12 can be vertically moved by the bellows 11 which controls the distance between plasma excitation electrodes 4 and 8.
The susceptor electrode 8 is connected to a second RF generator 15 via the shaft 13 and a matching circuit accommodated in a matching box 14. The chamber wall 10 and the susceptor shield 12 have the same DC potential.
FIG. 41 illustrates another example of a conventional plasma processing apparatus. Unlike the plasma processing apparatus shown in FIG. 42, the plasma processing apparatus shown in FIG. 41 is of a single-frequency excitation type. In other words, RF power is supplied only to the cathode 4 and the susceptor electrode 8 is grounded. Moreover, the matching box 14 and the second RF generator 15 shown in FIG. 42 are omitted. The susceptor electrode 8 and the chamber wall 10 have the same DC potential.
In the above-described plasma processing apparatuses, power having a frequency of approximately 13.56 MHz is generally supplied to generate a plasma between the electrodes 4 and 8. A plasma process such as CVD, sputtering, dry etching, ashing, or the like is then performed using the plasma.
The path of the RF power supplied from the RF generator 1 during such a process is as follows: An RF current flows into the cathode 4 through the power supply section. The current then returns to the ground position of the RF generator 1 via the susceptor electrode 8 and a power return section. The power return section is a section which has the same DC potential as that of the chamber wall 10 and is connected to the grounded line of the RF generator 1. For example, the power return section includes the chassis 21 and the matching box 2.
Through quantitative analysis, the inventors have found that the impedance of the ground line connected to the chamber at a plasma exciting frequency is higher than the impedance of the power return path by two to three orders of magnitude. The process results were the same regardless of whether the ground line was provided or not, thereby confirming the validity of the finding.
However, in the conventional plasma processing apparatuses, the feeder section including the feed plate 3 and the path of RF power returning to the ground of the RF generator 1, i.e., the power return section, have a high inductance. As a result, the RF current flowing into the plasma generation space between the electrodes 4 and 8 is regulated, thereby possibly decreasing the amount of power fed to the plasma space and the density of the generated plasma, which is a problem.
Moreover, the feeding loss in the power return section is high due to RF resistance components such as the chassis 21 and matching box 2, which are made of aluminum plates. This may decrease the effective RF power consumed in the plasma generation space.
Furthermore, the feeding loss at the RF resistance components of the feed plate 3 is large, thereby decreasing the effective RF power consumed in the plasma generation space.
The feed plate 3 is not necessarily connected to the center of the plasma excitation electrode 4. When the feed plate 3 is not connected to the center of the plasma excitation electrode 4, drifting of the RF current may occur, thereby generating a density distribution of the generated plasma in the electrode surface direction. This may also result in variation in layer characteristics in the surface direction, such as the thickness of the layer deposited by a plasma process.
When the size of the plasma excitation electrode 4, i.e., the maximum length from the center of power feeding to the periphery of the electrode, is larger than the quarter wave of the plasma excitation frequency, standing waves occur, which is a problem. That is, a distribution in plasma density is generated, and the uniformity of the layer thickness and layer characteristics in the substrate surface direction is impaired.
The above-described problems of variation in plasma processing are particularly severe when a power of approximately 150 MHz is supplied, when the plasma excitation electrode 4 and the susceptor electrode 8 have a diameter of approximately 60 cm, and when a substrate to be processed has a diameter of approximately 50 cm.
Particularly in the process for making substrates for liquid crystal displays, the substrate size is large, and the conventional apparatuses suffer more acutely from the above problem than in semiconductor manufacturing processes.
Another possible cause of the variation in the plasma processing is changes over time of the plasma processing apparatuses. In order to prevent changes over time, changes arising from the feed plate 3 in the feeder section and changes arising from the power return section, including the chassis 21 and the matching box 2, must be suppressed to achieve stable and uniform plasma processing over time. When plasma processing is performed a plurality of times, the RF characteristics of the power feed section varies each time the plasma processing is performed, each time resulting in different plasma processing results.
This problem is particularly severe when the feed plate 3 is made of copper and is of a type whose shape can be changed without having to disconnect the feed plate 3 from the plasma excitation electrode 4, i.e., when the shape of the feed plate 3 can be changed before or after the plasma processing or maintenance. The problems are also severe when the feed plate 3 is oxidized.
Yet another cause of variation in the plasma processing is deterioration, such as oxidation, of the surfaces of the chassis 21 and the matching box 2. The deterioration is particularly severe when the plasma processing is performed many times.
Referring now to FIG. 38, in the conventional plasma processing apparatuses, the supply line 1A, which functions as both the power supply section through which an RF current is fed to the chamber and the power return section through which the current returns to the ground, has a portion 1S disposed on the floor GF or the ceiling GC. The floor GF and the ceiling GC serve as a ground position. Because the RF current leaks to the floor GF or the ceiling GC, i.e., the ground position, from the portion 1S of the supply line 1A, the power fed to the plasma space decreases, feeding loss, which causes reduction of the density of the generated plasma, increases, and the RF power effectively consumed in the plasma generation space decreases.
Referring to FIG. 38, the supply line 1A also has a portion 1T located in the vicinity of the matching box 2 or the chamber wall 10 of the plasma processing chamber 60. The matching box 2 and the chamber wall 10 of the plasma processing chamber 60 are DC-ground positions. Because an RF current leaks from the portion 1T of the supply line 1A to the matching box 2 or the chassis 21 having a ground potential, the power fed to the plasma space decreases, feeding loss, which causes reduction of the density of the generated plasma, increases, and the RF power effectively consumed in the plasma generation space decreases.
When the plasma excitation frequency supplied from the RF generator 1 is increased, the supply line 1A has a small impedance ZX corresponding to the capacitance component (loss capacitance) CX generated due to the fact that the floor GF or the ceiling GC has a ground potential. A small impedance ZX increases the loss current IX leaking to the floor GF or the ceiling GC having a ground potential. An increase in the plasma excitation frequency also increases the impedance Z0 of the supply line 1A. As a result, the current flowing into the apparatus decreases, and the rate of change in the loss current IX increases. In other words, an increase in the variation of the effective power due to the variation in the feeding loss occurs. This problem and the above problem of increased feeding losses may significantly affect the stability of the power fed to the plasma space. The improvement thereof is desired.
Referring now to FIG. 39, when the matching box 2 is disposed separately from the plasma processing chamber 60, and the output terminal of the power supply section is connected to the plasma excitation electrode 4 through the feed line 3A, which is a coaxial cable, an RF current leaks from a portion 3S of the feed line 3A placed on the floor GF or the ceiling GC to the ground potential, i.e., the floor GF or the ceiling GC. As a result, the power fed to the plasma space decreases, feeding loss, which causes reduction of the density of the generated plasma, increases, and the RF power effectively consumed in the plasma generation space decreases.
As shown in FIG. 39, the feed line 3A also includes a portion 3T in contact with or disposed close to the matching box 2, the chamber wall 10 of the plasma processing chamber 60, and the like which have a DC ground potential. Because an RF current leaks from the portion 3T to the matching box 2, the chassis 21, and the like, the power fed to the plasma space decreases, feeding loss, which causes reduction of the density of the generated plasma, increases, and the RF power effectively consumed in the plasma generation space decreases.
As in the supply line 1A, the feed line 3A also suffer from an increased rate of change arising from the changes in impedance due to the feeding losses when the plasma excitation frequency supplied from the RF generator 1 is increased. This problem and the above problem of increased feeding losses may significantly affect the stability of the power fed to the plasma space. The improvement thereof is desired.
In most cases, coaxial cables, i.e., the supply line 1A and the feed line 3A, are not permanently fixed. Referring to FIGS. 38 and 39, the position of the portions 1S and 3S may change during plasma processing and before and after maintenance. Such a change in position may change the reactance and the RF resistance components, which may eventually lead to a change in power fed to the plasma space and nonuniformity in the density of the generated plasmas. Thus, such changes in the position of the portions 1S and 3S should be prevented to suppress changes in plasma processing over time and to achieve uniform and stable plasma processing over time.
In particular, when plasma processing is performed many times, the position of the coaxial cable changes each time, resulting in a change of the RF characteristics and nonuniform plasma processing results.
Moreover, a difference in plasma processing may also arise between plural plasma processing apparatuses constituting one plasma processing system. Such a difference should be minimized in a plasma processing system.
In the conventional plasma processing apparatuses, the flexibility of positioning the power feed section including components from the matching box 2 to the plasma excitation electrode 4 is small. The flexibility is desired to be improved without degrading the RF characteristics.